Led driver adapted to electronic transformer

ABSTRACT

Disclosed is an LED driver adapted to an electronic transformer, where the LED driver can ensure that the electronic transformer meets minimum load current requirements, and operates during an entire AC period by clamping the minimum inductor current. By controlling the LED load current through a current stabilization control circuit, the LED load can operate with relatively high control accuracy and fast response speed. In addition, the LED driver can match various electronic transformers based on traditional circuit structures, and the LED load can operate without flicking.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201210173730.2, filed on May 28, 2012, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of power supplies, and morespecifically to an LED driver adapted to an electronic transformer.

BACKGROUND

Spotlights (e.g., MR16 lamps) are extensively used in many lightingapplications today. Usually, spotlights can include two parts: anelectronic transformer; and a high power consumption quartz lamp. Withincreasing light-emitting diode (LED) applications in the lightingfield, it has become an irreversible trend of replacing traditionalquartz lights with LED lights. However, because the output of anelectronic transformer in a traditional lighting circuit (e.g., forquartz lights) is typically a high-frequency low-voltage AC powersupply, while LED lamps require a constant DC current source, a mismatchproblem can occur between an input power supply and a load. Therefore,an effective LED driver may be needed for converting the high-frequencylow-voltage AC power supply to a constant DC current source for LEDapplications.

SUMMARY

In one embodiment, a light-emitting diode (LED) driver adapted to anelectronic transformer, can include: (i) a rectifier bridge and a powerstage circuit coupled between the electronic transformer and an LEDload, where the power stage circuit comprises an inductor and a powerswitch; (ii) a first control circuit configured to control the powerswitch to regulate a current of the inductor and to maintain an outputvoltage of the power stage circuit as substantially constant based on afirst sense signal and a first voltage feedback signal, where the firstsense signal represents the inductor current, and where the firstvoltage feedback signal represents the output voltage of the power stagecircuit; (iii) an inductor current clamping circuit in the first controlcircuit, where the inductor current clamping circuit comprises aclamping voltage that matches a holding current of the electronictransformer, where the inductor current clamping circuit is configuredto clamp the inductor current to the holding current when the inductorcurrent is less than the holding current; and (iv) a current stabilizingcontrol circuit configured to detect a current of the LED load togenerate a detection signal, and to maintain the LED load current assubstantially constant based on the detection signal.

Embodiments of the present invention can advantageously provide severaladvantages over conventional approaches. For example, particularembodiments can provide an LED driver adapted to an electronictransformer. The LED driver can ensure that the electronic transformermeets minimum load current requirements, and functions during an entireAC period by clamping the minimum inductor current. In this way, the LEDdriver can achieve a relatively high utilization rate, without causingLED light flicker. Also, a relatively large electrolytic capacitor maynot be required after a rectifier bridge, so as to avoid possibledamages caused by impulse current. Other advantages of the presentinvention may become readily apparent from the detailed description ofpreferred embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an example LED driver.

FIG. 2 shows a schematic diagram of a first example LED driver adaptedto an electronic transformer in accordance with embodiments of thepresent invention.

FIG. 3 shows a schematic diagram of a second example LED driver adaptedto an electronic transformer in accordance with embodiments of thepresent invention.

FIG. 4 shows a schematic diagram of a third example LED driver adaptedto an electronic transformer in accordance with embodiments of thepresent invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set fourth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

A schematic diagram of an example light-emitting diode (LED) driver isshown in FIG. 1. A low-voltage AC power supply generated through anelectronic transformer can be rectified by a rectifier bridge, and thenfiltered by electrolytic capacitor CE1, to finally supply a constantoperating current for an LED by a current stabilizing control circuit.However, a relatively large electrolytic capacitor CE1 (e.g., hundredsof μF) may be placed after the rectifier bridge in the example ofFIG. 1. The existence of this electrolytic capacitor may turn theoriginal resistive load to a relatively large capacitive load for theelectronic transformer, and a relatively large impulse current may begenerated at the output current of the electronic transformer. As aresult, the impulse current may not only affect normal operation of theelectronic transformer, but can also increase circuit conduction loss,circuit temperature, and may reduce the operating lifetime.

In addition, a capacitive load of several hundred μF may cause theelectronic transformer to operate in an intermittent state to reduce theutilization rate of the electronic transformer. Further, ripples on theelectrolytic capacitor may increase to add a relatively large powerfrequency ripple on the LED current, which can lead to flicker of theLED lamp load. Therefore, matching the electronic transformer withvarious loads is an urgent problem in LED driver design. Because theelectronic transformer is typically designed for resistive loads, aminimum load current should be maintained during the entire AC period inorder to keep the transformer working normally. If the load currentdrops to lower than the minimum load current, or a large load currenttransient makes the load current lower than the minimum load current,the electronic transformer may turn off during the AC period to causeLED lamp flicking.

Therefore, particular embodiments are directed to a well-adapted LEDdriver for an electronic transformer to eliminate impulse currentwithout utilizing a relatively large electrolytic capacitor after arectifier bridge, and to ensure the electronic transformer operatesunder various types of loads without causing lamp flicking. For example,particular embodiments can provide an LED driver adapted to anelectronic transformer. The LED driver can ensure the electronictransformer meets minimum load current requirements, and operatesnormally during an entire AC period by clamping the minimum inductorcurrent. In this way, the LED driver can achieve a relatively highutilization rate, without causing LED light flicker. Also, a relativelylarge electrolytic capacitor may not be needed after a rectifier bridge,which can avoid possible damages caused by impulse current.

An LED driver adapted to an electronic transformer in particularembodiments can enable the electronic transformer to operate normallyunder substantially any circumstance by controlling the inductor currentthrough a control circuit and an inductor current clamping circuit, soas to achieve a high utilization ratio and two eliminate LED lightflicker. Also, a DC voltage source with a relatively small ripple can begenerated for a current stabilizing controlling circuit, and the LEDload current can be controlled by the current stabilizing controlcircuit to ensure lighting stability of the LED load. In addition, theLED driver can take advantage of relatively high accuracy and improvedresponse speed, and can match with any suitable electronic transformerunder different load conditions to ensure the electronic transformeroperates normally. The LED driver in particular embodiments can alsosave the relatively large electrolytic capacitor after the rectifierbridge to avoid a large impulse current from being added to the outputcurrent of the electronic transformer, to achieve a higher reliability.

In one embodiment, a light-emitting diode (LED) driver adapted to anelectronic transformer, can include: (i) a rectifier bridge and a powerstage circuit coupled between the electronic transformer and an LEDload, where the power stage circuit comprises an inductor and a powerswitch; (ii) a first control circuit configured to control the powerswitch to regulate a current of the inductor and to maintain an outputvoltage of the power stage circuit as substantially constant based on afirst sense signal and a first voltage feedback signal, where the firstsense signal represents the inductor current, and where the firstvoltage feedback signal represents the output voltage of the power stagecircuit; (iii) an inductor current clamping circuit in the first controlcircuit, where the inductor current clamping circuit comprises aclamping voltage that matches a holding current of the electronictransformer, where the inductor current clamping circuit is configuredto clamp the inductor current to the holding current when the inductorcurrent is less than the holding current; and (iv) a current stabilizingcontrol circuit configured to detect a current of the LED load togenerate a detection signal, and to maintain the LED load current assubstantially constant based on the detection signal.

Referring now to FIG. 2, shown is a schematic diagram of a first exampleLED driver adapted to an electronic transformer, in accordance withembodiments of the present invention. In this particular example, theLED driver can include a rectifier bridge and a power stage circuit thatcan connect between an electronic transformer and an LED load. The powerstage circuit can include inductor L1 and power switch Q1. Inductor L1can connect to the rectifier bridge, and filter capacitor C1 can connectto output terminals of the rectifier bridge to filter high-frequencyharmonic components of the output voltage of the rectifier bridge. Forexample, filter capacitor C1 can be any suitable capacitor type (e.g.,ceramic capacitor, film capacitor, polypropylene capacitor, etc.) with arelatively low capacitance to avoid relatively large impulse currents.

As shown in the example of FIG. 2, the power stage circuit can alsoinclude rectifier diode D1, output capacitor C2, and dividing resistorsR2 and R3. The power stage circuit in this example can be used to boostvoltage for the follow-on circuits. According to FIG. 2, those skilledin the art will recognize that the load current of the electronictransformer can represent the inductor current.

The LED driver can also include control circuit 21 and currentstabilizing control circuit 22. In this example, control circuit 21 canadjust the current of inductor L1 by controlling a duty cycle of powerswitch Q1, through which the load current of the electronic transformercan meet minimum load current requirements, and the output voltage ofthe power stage circuit can be maintained as substantial constant.Current stabilizing control circuit 22 can be used to keep the LED loadcurrent substantially constant. Example circuit structure and operationof control circuit 21 and current stabilizing control circuit 22 will bedescribed in more detail below.

Control circuit 21 can include inductor current clamping circuit 201,voltage control circuit 202, and current control loop 203. Voltagecontrol circuit 202 can include transconductance amplifier 202-1 and acompensation circuit, such as compensation capacitor C3. The invertinginput terminal of transconductance amplifier 202-1 can receive voltagefeedback signal V_(fb1) that represents an output voltage of the powerstage circuit. The non-inverting input terminal of transconductanceamplifier 202-1 can receive reference voltage signal V_(ref1), and theoutput signal of transconductance amplifier 202-1 can be used togenerate voltage control signal V_(comp) after being compensated by thecompensation circuit.

Inductor current clamping circuit 201 in this example can include diodeD2 and voltage source V. The cathode of diode D2 can connect tocompensation capacitor C3, the anode of diode D2 can connect to voltagesource V_(n), and the other terminal of voltage source

V_(n) can connect to ground. Inductor current clamping circuit 201 mayhave a clamping voltage that matches a holding current of the electronictransformer. For example, the clamping voltage can be V_(n)-V_(th),where V_(th) can be the conduction voltage drop of diode D2. Inductorcurrent clamping circuit 201 can receive voltage control signalV_(comp). When voltage control signal V_(comp) is less than the clampingvoltage, inductor current clamping circuit 201 can clamp voltage controlsignal V_(comp) to the clamping voltage, and the clamping voltage can betransferred to current control loop 203 as reference voltage V_(ref).

When voltage control signal V_(comp) is larger than the clampingvoltage, voltage control signal V_(comp) can be transferred to thecurrent control loop 203 as reference voltage V,_(f). What should benoted here is that voltage source V_(n) can be set according to theminimum load current of the electronic transformer. For example, whenthe minimum load current of the electronic transformer is relativelylarge, voltage source V_(n) can be relatively large to ensure theinductor current to be no less than the minimum load current of theelectronic transformer, and vice versa. Therefore, the clamping voltageof inductor current clamping circuit 201 can match the holding currentof the electronic transformer. Those skilled in the art will recognizethat the particular implementation of inductor current clamping circuit201 may not be limited to the example shown, and any suitable clampingcircuit can also be applied in the LED driver of particular embodiments.

Current control loop 203 can include comparator 203-1 and flip-flop203-2. The inverting input terminal of comparator 203-1 can receivereference voltage signal V_(ref), and the non-inverting input terminalof comparator 203-1 can receive sense signal V_(sen1) that representsthe inductor current. Comparator 203-1 can generate comparison signalV_(r1) after comparing reference voltage signal V_(ref) against sensesignal V_(sen1). Flip-flop 203-2 can receive comparison signal V_(r1) atits reset terminal, and clock signal V_(p1) at its set terminal, and maygenerate control signal V_(c1). For example, clock signal V_(p1) can bea pulse signal with fixed frequency, and control signal V_(c1) can beused to control power switch Q₁. In this way, the inductor current canbe adjusted.

When clock signal V_(p1) changes to a high level, flip-flop 203-2 canoutput control signal V_(i) at its output terminal Q as a high levelstate. Control signal V_(c1) can turn on power switch Q1 to increase thecurrent of inductor L1. Then, the inductor current can rise at a certainslope rate. When sense signal V_(sen1) increases beyond referencevoltage V_(ref), comparison signal V_(r1) output by comparator 203-1 canchange to a high level at the reset terminal of flip-flop 203-2. Thus,control signal V_(e1) output at terminal Q of flip-flop 203-2 can bechanged to be low level. Also, power switch Q1 can be turned off by thecontrol signal V_(e1) to reduce the current of inductor L1. By thiscycle, the inductor current can be controlled.

For example, current stabilizing control circuit 22 can include erroramplifier 204, comparator 205, flip-flop 206, inductor L2, and powerswitch Q2. Inductor L2 and power switch Q2 can be series connectedbetween the LED load and ground. In addition, current stabilizingcontrol circuit 22 can also include freewheel diode D3 and resistor R5.Error amplifier 204 can receive a detection signal that represents theLED load current, and reference voltage V_(ref2). Specifically, thedetection signal in this example can be average voltage signal V_(avg)that represents the average value of the LED load current.

Error amplifying signal V_(err) can be output by error amplifier 204based on average voltage signal V_(avg) and reference voltage V_(ref2).Comparator 205 can receive error amplifying signal V_(err) at itsinverting input terminal, and a sense signal (e.g., peak voltage signalV_(peak) that represents the peak current of inductor L2 current) at itsnon-inverting input terminal. Comparator 205 may generate comparisonsignal V_(r2). When peak voltage signal V_(peak) reaches a level oferror amplifying signal V_(err), comparison signal V_(r2) may go high.Because the reset terminal of flip-flop 206 can receive comparisonsignal V_(r2), control signal V_(c2) can be generated to turn off powerswitch Q2.

The set terminal of flip-flop 206 can receive clock signal V_(p2). Forexample, clock signal V_(p2) can be a pulse signal with fixed frequency.When clock signal V_(p2) is high, flip-flop 206 can generate controlsignal V_(c2) to turn on power switch Q2. By this cycle, the LED loadcurrent can be controlled substantially constant.

According to the example operation above, a control scheme of particularembodiments can adjust a reference voltage signal through an inductorcurrent clamping circuit, and the voltage control circuit underconditions when LED loads may require different driving power supplies.On the one hand, when a required driving power supply of the LED load isrelatively large, the reference voltage can be relatively high so as tomake the inductor current large enough to satisfy the requirements ofnormal operation of the LED loads.

On the other hand, when the required driving power supply is relativelylow, the reference voltage can be held at a predetermined value toensure that the inductor current can meet the minimum load currentrequirement of the electronic transformer, so as to make sure that theelectronic transformer can avoid flicker of the LED load. In this way,the electronic transformer and LED load can be well matched undervarious circumstances by the control circuit of particular embodiments.Also, the current stabilizing control circuit of particular embodimentscan respond to LED load current changes relatively quickly, and canmaintain the LED load current as substantially constant. The currentstabilizing control circuit can take advantage of relatively highcontrol accuracy and relatively fast response speeds, as compared toconventional approaches.

Referring now to FIG. 3, shown is a schematic diagram of a secondexample LED driver adapted to an electronic transformer, in accordancewith embodiments of the present invention. As compared with the exampleshown in FIG. 2, the particular example of FIG. 3 can includeovervoltage protection circuit 33. Overvoltage protection circuit 33 canbe used to maintain the output voltage of the power stage assubstantially constant, so as to provide a substantially stable voltagesource for current stabilizing control circuit 22.

For example, overvoltage protection circuit 33 can include hysteresiscomparator 307, where the inverting input terminal can receive voltagefeedback signal V_(fb2) that represents the output voltage of the powerstage circuit. The non-inverting input terminal of hysteresis comparator307 can receive reference voltage signal V_(n), and hysteresiscomparator 307 can output hysteresis comparison signal V_(ii) ^(,).Hysteresis comparison signal V_(r1), and control signal V_(c1) can beused to generate pulse-width modulation (PWM) control signal PWM1 tocontrol power switch Q1 through the logic operation of AND-gate 308.

When voltage feedback signal V_(fb2) is higher than upper thresholdvoltage V_(H1) of hysteresis comparator 307, hysteresis comparisonsignal V_(r1), can be low. Also, AND-gate 308 can generate PWM controlsignal PWM1 to maintain power switch Q1 in an off state. As a result,the output voltage may decrease gradually. When voltage feedback signalV_(fb2) is decreased to the lower threshold voltage V_(L1) of hysteresiscomparator 307, hysteresis comparison signal V_(r1), can go high. Also,AND-gate 308 can generate PWM control signal PWM1 to control powerswitch Q1 according to control signal V_(c1). Thus, the output voltagemay increase gradually until voltage feedback signal V_(fb2) reaches alevel of the upper threshold voltage V_(H1) of hysteresis comparator307. By repeating the operation cycle, the output voltage ripple of thepower stage can be relatively small, and the output voltage can besubstantially constant to ensure good operation stability andreliability of the follow-on current stabilizing control circuit.

By applying the above-described LED driver adapted to an electronictransformer in accordance with particular embodiments, the followingfeatures can be achieved. For example, the control circuit can ensurethat the load current of the electronic transformer is no lower than arequired minimum load current. This can ensure that the electronictransformer functions in a normal operating state. Also, the currentstabilizing control circuit can be used to maintain the LED load currentas substantially constant. In addition, the overvoltage protectioncircuit can decrease the output voltage ripple of the power stage, so asto provide a substantially constant voltage source for the follow-onstage circuit.

Referring now to FIG. 4, shown is a schematic diagram of a third exampleLED driver adapted to an electronic transformer, in accordance withembodiments of the present invention. A difference in this example ascompared to those described above involves current stabilizing controlcircuit 42. For example, current stabilizing control circuit 42 caninclude hysteresis comparator 405. The inverting input terminal ofhysteresis comparator 405 can receive detection signal V_(s) thatrepresents the LED load current. The non-inverting input terminal ofhysteresis comparator 405 can receive reference voltage signal V_(ref4),and can generate hysteresis comparison signal V_(r2). Hysteresiscomparison signal V_(r2), and control signal V_(c1) can be used togenerate PWM control signal PWM2 to control power switch Q1 through thelogic operation of AND-gate 404.

Based on the operation principle of the hysteresis comparator, whendetection signal V_(s) is less than upper threshold voltage V_(H2) ofhysteresis comparator 405, hysteresis comparison signal V_(r2), can behigh. Also, AND-gate 404 can generate PWM control signal PWM2 to controlpower switch Q1 based on control signal V_(c1), and capacitor C2 can becharged to increase the output voltage. Consequently, the LED currentmay gradually increase.

When detection signal V_(s) reaches a level of upper threshold voltageV_(H2) of hysteresis comparator 405, hysteresis comparison signalV_(r2)′ may go low. Then, AND-gate 404 can generate PWM control signalPWM2 to turn off power switch Q1. Capacitor C2 can supply power for theLED load, and the LED load current may gradually decrease.

When detection signal V_(s) decreases to a level of lower thresholdvoltage V_(L) of hysteresis comparator 405, hysteresis comparison signalV_(r2), may go higher to turn on power switch Q1, so as to graduallyincrease the LED current. By repeating this cycle, the LED load currentripple can be controlled to be within a relatively small range, and theLED load current can be maintained as substantially constant.

The example LED driver shown in FIG. 4 can adapt to operatingrequirements of an electronic transformer by adjusting the current ofinductor L1 through the control circuitry (e.g., control circuit 21 andcurrent stabilizing control circuit 42). Also, the structure and controllogic of the current stabilizing control circuit can be relativelysimple so as to be utilized in applications that require relatively lowload accuracy, and may also offer lower associated product costs.

An LED driver adapted to an electronic transformer in particularembodiments can adjust the electronic transformer load current bycontrol circuitry to ensure the electronic transformer operatesnormally. In this way, LED loads can meet requirements of differentapplications, and the LED loads can match well with electronictransformers to avoid light flicking scene in traditional circuits.Also, the overvoltage protection circuit in particular embodiments canprovide a DC voltage source with relatively low voltage ripple for thefollow-on current stabilizing control circuit. The current stabilizingcontrol circuit in particular embodiments can satisfy requirements(e.g., high accuracy, low product cost, etc.) of different applicationsto ensure suitable operation of various LED loads. In addition,particular embodiments can achieve relatively small ripple withoututilizing a large electrolytic capacitor after the rectifier bridge.

In particular embodiments, circuit structures and components of thevoltage control circuit, current control loop, inductor current clampingcircuit, current stabilizing control circuit, and so on, are not limitedto the examples shown and discussed above. Any suitable circuit with asame or similar function can be utilized in particular embodiments. Inaddition, the power stage circuit can also be other suitable topologystructures or converter types (e.g., single-ended primary-inductorconverter [SEPIC], etc.).

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to the particularuse contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

1. A light-emitting diode (LED) driver adapted to an electronictransformer, the LED driver comprising: a) a rectifier bridge and apower stage circuit coupled between said electronic transformer and anLED load, wherein said power stage circuit comprises an inductor and apower switch; b) a first control circuit configured to control saidpower switch to regulate a current of said inductor; and c) an inductorcurrent clamping circuit in said first control circuit, wherein saidinductor current clamping circuit is configured to regulate saidinductor current by controlling a duty cycle of said power switch tomaintain a load current of said electronic transformer to be no lessthan a holding current of said electronic transformer.
 2. The LED driverof claim 13, wherein said first control circuit further comprises: a) avoltage control circuit configured to generate said voltage controlsignal based on said output voltage of said power stage circuit and saidreference voltage, wherein when said voltage control signal is less thansaid clamping voltage, said clamping voltage is provided to a currentcontrol loop as a reference voltage signal; b) wherein when said voltagecontrol signal is greater than said clamping voltage, said voltagecontrol signal is provided to said current control loop as saidreference voltage signal; and c) said current control loop beingconfigured to receive a first sense signal that represents said inductorcurrent and said reference voltage signal, and to generate a firstcontrol signal to control said power switch.
 3. The LED driver of claim2, wherein said voltage control circuit comprises: a) a firsttransconductance amplifier configured to receive said first voltagefeedback signal and said first reference voltage signal; and b) acompensation circuit configured to receive an output from said firsttransconductance amplifier, and to generate said voltage control signal.4. The LED driver of claim 2, wherein said current control loopcomprises: a) a first comparator configured to receive said first sensesignal and said reference voltage signal, and to generate a firstcomparison signal; and b) a first flip-flop configured to receive saidfirst comparison signal at a reset terminal and a first clock signal ata set terminal, and to generate said first control signal to controlsaid power switch.
 5. The LED driver of claim 10, wherein said currentstabilizing control circuit comprises: a) a second inductor and a secondpower switch coupled between said LED load and ground; b) an erroramplifier circuit configured to receive said detection circuit and asecond reference voltage signal, and to generate a first erroramplifying signal; c) a second comparator configured to receive saidfirst error amplifying signal and said second sense signal, and togenerate a second comparator signal, wherein said second sense signalrepresents a second inductor current; and d) a second flip-flopconfigured to receive said second comparison signal at a reset terminaland a second clock signal at a set terminal, and to generate a secondcontrol signal to control said second power switch.
 6. The LED driver ofclaim 1, wherein said LED driver further comprises: a) an overvoltageprotection circuit having a first hysteresis comparator configured toreceive a second voltage feedback signal and a third reference voltagesignal, and to generate a first hysteresis comparison signal, whereinsaid second voltage feedback signal represents an output voltage of saidpower stage circuit; and b) a first AND-gate configured to receive saidfirst hysteresis comparison signal and a first control signal from saidfirst control circuit, and to generate a first pulse-width modulation(PWM) control signal to control said power switch.
 7. The LED driver ofclaim 10, wherein said current stabilizing control circuit comprises: a)a second hysteresis comparator configured to receive said detectionsignal and a fourth reference voltage signal, and to generate a secondhysteresis comparison signal; and b) a second AND-gate configured toreceive said second hysteresis comparison signal and a first controlsignal from said first control circuit, and to generate a second PWMcontrol signal to control said power switch.
 8. The LED driver of claim1, further comprising a filter capacitor coupled in parallel connectedto output terminals of said rectifier bridge, wherein said filtercapacitor is configured to filter high-frequency harmonic components ofan output voltage of said rectifier bridge.
 9. The LED driver of claim1, wherein said first control circuit is configured to control saidpower switch to regulate said inductor current to maintain an outputvoltage of said power stage circuit as substantially constant.
 10. TheLED driver of claim 1, wherein said LED driver further comprises acurrent stabilizing control circuit configured to maintain said LED loadcurrent as substantially constant based on a detection signal thatrepresents a current of said LED load.
 11. The LED driver of claim 1,wherein said inductor current clamping circuit is configured to providea clamping voltage that matches said holding current of said electronictransformer.
 12. The LED driver of claim 11, wherein a voltage controlsignal that represents an error between an output voltage of said powerstage circuit and a reference voltage is clamped to be not less thansaid clamping voltage.
 13. The LED driver of claim 12, wherein saidinductor current clamping circuit comprises a voltage source coupled toa diode to provide said clamping voltage.
 14. A method of controlling alight-emitting diode (LED) driver adapted to an electronic transformer,the LED driver comprising a rectifier bridge and a power stage circuitcoupled between said electronic transformer and an LED load, whereinsaid power stage circuit comprises an inductor and a power switch, themethod comprising: a) controlling, by a first control circuit, saidpower switch to regulate a current of said inductor; and b) regulating,by an inductor current clamping circuit in said first control circuit,said inductor current by controlling a duty cycle of said power switchto maintain a load current of said electronic transformer to be no lessthan a holding current of said electronic transformer.
 15. The method ofclaim 14, further comprising filtering, by a filter capacitor coupled inparallel connected to output terminals of said rectifier bridge,high-frequency harmonic components of an output voltage of saidrectifier bridge.
 16. The method of claim 14, further comprisingcontrolling, by said first control circuit, said power switch toregulate said inductor current to maintain an output voltage of saidpower stage circuit as substantially constant.
 17. The method of claim14, further comprising providing, by said inductor current clampingcircuit, a clamping voltage matching said holding current of saidelectronic transformer.
 18. The method of claim 17, further comprisingclamping a voltage control signal that represents an error between anoutput voltage of said power stage circuit and a reference voltage to benot less than said clamping voltage.
 19. The method of claim 18, furthercomprising providing, by said inductor current clamping circuit, saidclamping voltage using a voltage source coupled to a diode.
 20. Themethod of claim 19, further comprising: a) generating, by a voltagecontrol circuit, said voltage control signal based on said outputvoltage of said power stage circuit and said reference voltage; b)providing said clamping voltage to a current control loop as a referencevoltage signal when said voltage control signal is less than saidclamping voltage; c) providing said voltage control signal to saidcurrent control loop as said reference voltage signal when said voltagecontrol signal is greater than said clamping voltage; and d) generating,by said current control loop being configured to receive a first sensesignal that represents said inductor current and said reference voltagesignal, a first control signal to control said power switch.